The present invention relates generally to a concentrator system having a master station connected to a public switched tele-communications network and a terminal station that connects remote user terminals through commonly shared two-way channels to the public switched network, and more specifically to a concentrator system in which the terminal station locally establishes intraoffice connections for saving the channel resource by clearing the channels which were used to establish initial connections to the network to activate tariff charges on the calling party of the intraoffice call.
In most cases, the terminal station of the concentrator system is located in a remote, scarcely populated area where power supplies are not available. Solar collectors with backup storage batteries are employed for such terminal stations. However, the climate dependent nature of the solar battery system often results in a failure to keep the power supply voltage to the rated value. In addition, the long trip to the remote terminal station and the exposure to the weather can result in a tendency that the equipment receives less maintenance attention, causing equipment failures to occur. For channel saving purposes for local (or intraoffice) calls between subscribers of the same terminal station, connections are initially established through the common channel to the public switched network to initiate tariff charges on the calling party and then cleared, and local connections are reestablished in the terminal station to continue the call. If a trouble should occur in the terminal station, this will likely to result in the local connections being automatically cleared, interrupting the local call. However, the central station is not notified of this fact due to its failure to receive an end-of-call signal from the terminal station and the public switched network continues the operation of the tariff counter of the calling party. The counter will therefore be incremented indefinitely, and an undesirable situation results.